Internet Engineering Task Force R. Winter
Internet-Draft NEC Laboratories Europe
Intended status: Informational M. Faath
Expires: September 22, 2016 University of Applied Sciences Augsburg
A. Ripke
NEC Laboratories Europe
March 21, 2016
Multipath TCP Support for Single-homed End-systemsdraft-wr-mptcp-single-homed-07
Abstract
Multipath TCP relies on the existence of multiple paths between end-
systems. These are typically provided by using different IP
addresses obtained by different ISPs at the end-systems. While this
scenario is certainly becoming increasingly a reality (e.g. mobile
devices), currently most end-systems are single-homed (e.g. desktop
PCs in an enterprise). It seems also likely that a lot of network
sites will insist on having all traffic pass a single network element
(e.g. for security reasons) before traffic is split across multiple
paths. This memo therefore describes mechanisms to make multiple
paths available to multipath TCP-capable end-systems that are not
available directly at the end-systems but somewhere within the
network.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 22, 2016.
Winter, et al. Expires September 22, 2016 [Page 1]

Internet-Draft single-homed MPTCP March 2016
In order to illustrate the general mechanism we make use of a simple
reference scenario shown in Figure 1.
+-------+
| DHCP |
+-------+ +----------+ Server|
| | | | |
| Host +------+ +-------+
| | | +-------+ ISP 1
+-------+ +------+ |----------
| Gatew.|
| |----------
+-------+ ISP 2
Figure 1: Reference Scenario
The scenario in Figure 1 depicts e.g. a possible SOHO or enterprise
setup where a gateway/router is connected to two ISPs and a DHCP
server gives out leases to hosts connected to the local network.
Note that both, the gateway and the DHCP server could be on the same
device (similar to current home gateway implementations). Also, the
two ISPs could really be two different access technologies (e.g. LTE
and DSL) provided by a single ISP.
The host is running a multipath-capable IP stack, however it only has
a single interface. The methods described in the following sections
will let the host make use of the gateway's two interfaces without
requiring modifications to the MPTCP implementation.
2. Approaches to Use Multiple Paths in the Network
All approaches in this document do not require changes to the wire
format of MPTCP and both communicating hosts need to be MPTCP-
capable. The benefit this approach has is that a) it has no
implications on MPTCP standards, b) it will hopefully encourage the
deployment of MPTCP as the number of scenarios where MPTCP brings
benefits vastly increases and c) these approaches do not require
complex middle-boxes to implement MPTCP-like functionality in the
network as other approaches have suggested before.
2.1. Exposing Multiple Paths Through End-host Auto-configuration
Multipath TCP distinguishes paths by their source and destination IP
addresses. Assuming a certain level of path diversity in the
Internet, using different source and destination IP addresses for a
given subflow of a multipath TCP connection will, with a certain
probability, result in different paths taken by packets of different
subflows. Even in case subflows share a common bottleneck, the
Winter, et al. Expires September 22, 2016 [Page 3]

Internet-Draft single-homed MPTCP March 2016
proposed multipath congestion control algorithm [RFC6356] will make
sure that multipath TCP will play nicely with regular TCP flows.
In order to not require changes to the TCP implementation, we keep
the above assumptions multipath TCP makes, i.e. working with
different IP addresses to use different paths. Since the end-system
is single-homed, all IP addresses are bound to the same physical
interface. In our reference scenario in Figure 1, the host would
e.g. receive more than one RFC1918 [RFC1918] private IP address from
the DHCP server as depicted in Figure 2.
Host Gateway
+-----------------+ ISP1
+--------+ | src. |
| virt. | 10.1.2.5 | 10.1.0.0/16 __.+----------
| +---+ | __.--' |
| phys. | | | __.--' N |
| +----------+.:_ A |
| | 10.2.2.6 | `-.._ T |
+--------+ | src. `-.._ | ISP2
| 10.2.0.0/16 `-..+----------
| |
+-----------------+
Figure 2: Gateway internals
The gateway that is shown in Figure 2 has received two IP addresses,
one from each ISP that it is connected to (ISP1 and ISP2). The NAT
that the gateway is implementing needs to "map" each private IP
address of the host consistently to a one of the addresses received
by the ISPs, i.e. each private IP to a different public IP. Packets
sent by the host to the gateway are then routed based on the source
address found in the packets as illustrated in the figure. In other
words, depending on the source address of the host, the packets will
either go through ISP 1 or ISP 2 and TCP will balance the traffic
across those two links using its built-in congestion control
mechanism.
The way the gateway has received its public IP addresses is not
relevant. It could be via DHCP, IPCP or static configuration. In
order to configure the hosts behind the gateway, we propose to make
use of provisioning domains [RFC7556], more specifically one
provisioning domain per external gateway interface (the two
interfaces to ISP1 and ISP2 in Figure 2). The DHCPv6 specification
for encoding provisioning domains can be found in
[I-D.ietf-mif-mpvd-dhcp-support].
Winter, et al. Expires September 22, 2016 [Page 4]

Internet-Draft single-homed MPTCP March 2016
In order to signal to the host, that each provisioning domain will
result in a different path towards the Internet, this memo introduces
a new DHCP option called EXT_ROUTE, which will be included in each
provisioning domain sent by the server. The option value will
determine which external interface is used to sent the traffic when
using the configuration information present in the respective
provisioning domain.
Upon receipt of a DHCP offer including multiple provisioning domains,
or multiple offers each including one or more provisioning domains,
the client SHOULD create up to n virtual interfaces, where n is one
less than the number of different EXT_ROUTE option values found in
all received provisioning domains. Each virtual interface will
contact the DHCP server and will request configuration information
for the respective provisioning domains, excluding the configuration
of the physical interface.
2.2. Heuristic Use of Multiple Paths
The auto-configuration mechanism above has the advantage that
available paths and information on how to use them are directly sent
to the end-host. In other words, there is an explicit signalling of
the availability of multiple paths to the end-host. This has the
advantage that the host can efficiently use these paths.
This method works well when multiple paths are available close to the
end-host and means for auto-configuration are available. But that is
not always the case. Another method to use different paths in the
network without prior knowledge of their existence is to apply
heuristics in order to exploit setups where Equal Cost Multi-path
[RFC2991], a widely deployed technology [ECMP_DEPLOYMENT], or similar
per-flow load-balancing algorithms are employed.
The ADD_ADDR option defined in [RFC6824] can be used to advertise the
same address but a different port to open another subflow.
Additionally, the MP_JOIN option can also be used to open another
subflow with the same IP address and e.g. a different source port
given that a different address ID is used. This means there are
multiple scenarios possible (e.g. either sender-initated or receiver-
initiated) where single-homed end-hosts can influence the 5-tuple
(source and destination IP addresses and port numbers plus protocol
number) which is often used as the basis for per-flow load balancing.
Changing the 5-tuple will only with a certain probability result in
using a different path unless the load-balancing algorithm that is
used is known to the MPTCP implementation (an assumption we cannot
generally make). This means that a number of subflows might end up
on the same path. Fortunately, the MPTCP congestion control
Winter, et al. Expires September 22, 2016 [Page 5]

Internet-Draft single-homed MPTCP March 2016
algorithm will make sure that the collection of subflows on that path
will not be more agressive than a single TPC flow.
3. Other scenarios and extensions
The reference scenario is only one conceivable setting. Other
scenarios such as DSL broadband customers or mobile phones are
conceivable as well. As an example, take the DSL scenario. The home
gateway could be provided with multiple IP addresses using extensions
to IPCP. The home gateway in turn can then implement the DHCP server
and gateway functionality as described before. More scenarios will
be described in future versions of this document.
4. Alternative approaches
One alternative is that a DHCP server always sends n offers, where n
is the number of interfaces at the gateway to different ISPs. The
client could then accept all or a subset of these offers. This
approach seems interesting in environments where there are multiple
DHCP servers, one for each ISP connection (think multiple home
gateways). However, accepting multiple offers based on a single DHCP
request is not standard's compliant behavior (at least for the DHCPv4
case). Also, to cater for a scenario that only contains a single
DHCP server, server changes are needed in any case. Finally, correct
routing is not always guaranteed in these scenarios.
An interesting alternative is the use of ECMP at the gateway for load
distribution and let MPTCP use different port numbers for subflows.
Assuming that ECMP is available at the gateway, this approach would
work fine today. The only drawback of the approach is that it
involves a little trial and error to find port numbers that actually
hash to different paths used by ECMP [RFC2991].
5. Acknowledgements
Part of this work was supported by Trilogy (http://www.trilogy-project.org), a research project (ICT-216372) partially funded by the
European Community under its Seventh Framework Program. The views
expressed here are those of the author(s) only. The European
Commission is not liable for any use that may be made of the
information in this document.
6. IANA Considerations
One new DHCP options is required by this version of this document.
Winter, et al. Expires September 22, 2016 [Page 6]